CHAPTER V. 



DISJUNCTIVE JPllOPOSITiONS. 



IN the previous chapter I have exhibited various cases 

 of deductive reasoning by the process of substitution, avoid- 

 ing the introduction of disjunctive propositions ; but we 

 cannot long defer the consideration of this more complex 

 class of identities General terms arise, as Ave have seen 

 (p. 24), from classifying or mentally uniting together all 

 objects which agree in certain qualities, the value of this 

 union consisting in the fact that the power of knowledge 

 is multiplied thereby. In forming such classes or general 

 notions, we overlook or abstract the points of difference 

 which exist between the objects joined together, and fix our 

 attention only on the points of agreement. But every 

 process of thought may be said to have its inverse process, 

 which consists in undoing the effects of the direct process. 

 Just as division undoes multiplication, and evolution un- 

 does involution, so we must have a process which undoes 

 generalization, or the operation of forming general notions. 

 This inverse process will consist in distinguishing the 

 separate objects or minor classes which are the constituent 

 parts of any wider class. If we mentally unite together 

 certain objects visible in the sky and call them planets, we 

 shall afterwards need to distinguish the contents of this 

 general notion, which we do in the disjunctive proposi- 

 tion 



A planet is either Mercury or Venus or the Earth or 



or Neptune. 



Having formed the very wide class " vertebrate animal," 

 we may specify its subordinate classes thus : " A verte- 



